1. Field of the Invention
The present invention pertains to heart valve prostheses and in particular, to bi-leaflet heart valve prostheses using pivotable valve members.
2. Description of Related Art
Various types of heart valve prostheses have been proposed, and many give generally satisfactory operation. One popular design for a heart valve prosthesis includes an annular valve body in which a pair of opposed leaflet occluders are pivotally mounted. The occluders are movable between a closed, mated position, blocking blood flow in an upstream direction, thereby minimizing regurgitation, and an open position, allowing blood flow in downstream direction.
Because hemodynamic energy alone is relied upon for proper operation of the heart valve between its open and closed positions, it is generally desirable to reduce dynamic friction losses in the heart valve which would needlessly burden the cardiac system. Static frictional forces are not significant in a well designed heart valve if there are no surfaces normal to the leaflet's initial opening movement. A complete reversal of the flow direction initiates movement of the occluder away from the surfaces that constrain it from rotation. A certain amount of play in the pivot mechanism is necessary to assure that relatively small and fragile ends of the pivots do not bear the high loads the valve is exposed to when it is fully closed.
Over the countless number of operations of a heart valve, the projections and depressions are subjected to wear. For the reasons set forth in U.S. Pat. No. 4,689,046, to Bokros, also assigned to the Assignee of this Application, spherical projections and depressions are susceptible to significant amounts of "play" in directions lying in the plane of the leaflet and extending generally perpendicular to the diametrical edge thereof. It has been observed that relatively small amounts of wear adjacent the tip of the spherical projection or the corresponding center portion of the recess results in a significant amount of lateral play, even for relatively minute amounts of increased "end play", that is, in directions generally parallel to the diametrical leaflet edge and extending along the hinge points of a leaflet. As a result of this lateral play, the motion and the sequence, especially the synchronous cooperation of the leaflets, becomes less well defined. As a result, performance of the leaflet may become erratic, as is evidenced, for example, by an asynchronous closure of the valve. While prior art heart valves have generally proven to be very reliable, and to have a projected life expectancy exceeding that of the patient, it is desirable to achieve increased margins of safety by providing a prosthesis which substantially exceeds reliability and performance requirements.
Several other improvements to heart valve prostheses are also desired. For example, it is desirable to impart a more rapid closing time to the leaflet occluders so as to reduce regurgitation. However, such quickening of the closing time should not be accompanied by an increase in noise during operation of the prosthesis, for example, as the leaflet occluders seat against the valve body to block regurgitation. Also, any rebounding of the leaflets should be controlled so as to prevent unnecessary wear, and to conserve hemodynamic energy. Any decrease in valve closing time, therefore, should not contribute to rebounding of the leaflets.
In the past, leaflet occluders have occasionally been slightly undersized so as to allow a purging blood flow around them, even when the leaflets are closed. Such flows wash over edge surfaces of the leaflets and the valve body to prevent clotting that might occur at those locations. The hemodynamic energy of a patient, however, should be conserved. Accordingly, the amount of undersizing of the leaflet occluders must be accurately controlled. Such sizing, of course, depends upon the manufacturing tolerances which can be obtained for the selected geometry of the valve body as well as of the leaflet occluders. Since manufacturing costs are directly related to manufacturing tolerances, alternative arrangements for providing a purging flow around the leaflet occluders, particularly at their hinged connections to the valve body, in a manner which conserves hemodynamic energy, are still being sought.